1. Technical Field
The present invention relates to a measuring device for determining the moisture in gases.
2. Description of Related Art
In measuring moisture in gases, a distinction is made between absolute humidity and relative humidity. Absolute humidity provides the mass of evaporated liquid per gas volume unit (normally in g/m3). Relative humidity is the absolute humidity divided by the saturation humidity, the latter being the maximum possible mass of evaporated liquid per gas volume unit. Saturation humidity is thus a function of the temperature.
The use of capacitive moisture sensors containing a capacitor, whose capacitance varies as a function of the moisture of the gas surrounding the sensor, is known for measuring moisture in gases. Problems may occur in such sensors, in particular at high moisture levels.
There is the danger of moisture precipitating on the sensor at high moisture levels if the temperature of the moisture sensor is lower than the dew point of the surrounding gas. This results in the measured quantity being corrupted and may also result in damage to the moisture sensor if the surrounding contains chemical components which form an acid or a base on the sensor in the presence of water.
A drift in the measured quantities in the event of long-term use in an environment having a high moisture level, resulting in corruption of the measured quantities, represents another problem with conventional capacitive moisture sensors.
German Patent 28 51 686 C2 teaches that these problems may be eliminated by heating the moisture sensor to keep it above the temperature of the gas. In addition to a measured quantity delivered by the sensor, the ambient temperature and the temperature of the sensor are needed for determining the relative humidity, for which additional measurements and measuring devices are needed. In addition, a complex regulating circuit is required for this known device.
It is furthermore known from WO 97/02468 that a moisture sensor may be kept at a constant temperature, which is higher than the temperature of the gas, by heating. The known device is used for determining the absolute humidity in a gas, for which the absolute temperature of the gas, in addition to a measured quantity delivered by a moisture sensor, is needed. For this purpose, an additional measurement and an additional measuring device are required. In addition, the operating range is limited due to the constant operating temperature of the sensor. Measuring moisture without the danger of moisture formation on the sensor is only possible if the temperature of the gas is lower than the operating temperature of the sensor. In order to be able to determine the relative humidity of the gas using this known measuring device known from WO 97/02468, a correction of the measured absolute humidity value must be calculated.